Retrofit Methodology
A detailed walkthrough of our PAS2035 retrofit methodology — from whole-dwelling assessment through improvement option evaluation to installation oversight and post-retrofit monitoring. Every stage follows PAS2035:2023.
Retrofit Assessment (PAS2035 Stage 1)
The foundation of every retrofit project — a whole-dwelling assessment that captures fabric, ventilation, heating, energy systems and household context.
- ▸Whole-dwelling survey: wall construction, insulation, floor type, roof/loft, windows and doors
- ▸Ventilation assessment: existing strategy, extract fans, trickle vents, background ventilation rates
- ▸Heating system audit: boiler type, efficiency, controls, heat distribution and hot water system
- ▸Moisture risk assessment: identify interstitial condensation and moisture management risks pre-retrofit
- ▸Household occupancy, heating patterns and affordability data captured for occupancy-adjusted modelling
- ▸U-value estimation from construction age and build-up, validated with thermography where required
Improvement Option Evaluation & Medium-Term Plan (Stage 2)
Retrofit measures are evaluated against HEM metrics, EPC targets, funding eligibility and technical feasibility to produce a prioritised medium-term improvement plan.
- ▸Fabric-first measure evaluation: loft, wall and floor insulation, window upgrades, air-tightness improvements
- ▸Heating decarbonisation options: heat pump feasibility, district heating connection, hybrid systems
- ▸Ventilation strategy: trickle vent upgrades, mechanical ventilation with heat recovery (MVHR) where appropriate
- ▸Each measure modelled for HEM score impact, CO₂ reduction, capital cost and payback period
- ▸Medium-term improvement plan sequenced to avoid measure conflicts (e.g. insulation before heating upgrade)
- ▸Funding eligibility check: SHDF wave, ECO4, Great British Insulation Scheme and local grant programmes
Design & Specification (Stage 3)
Selected measures are specified to PAS2035:2023 standards with full technical design, U-value targets and installation quality requirements.
- ▸Fabric specification with target U-values aligned to Building Regulations Part L and PAS2035 requirements
- ▸Ventilation design: background, purge and mechanical ventilation strategy compliant with Approved Document F
- ▸Heating system design: heat loss calculations, emitter sizing, flow temperatures and control strategy
- ▸Thermal bridging assessment and psi-value specification to minimise heat loss at junctions
- ▸Moisture-safe design: WUFI or dew-point analysis where insulation changes the thermal envelope
- ▸Risk register: technical, moisture, fire and resident impact risks identified and mitigated
Installation Oversight & Monitoring (Stages 4–5)
We provide retrofit coordination oversight during installation and post-retrofit monitoring to verify real-world performance and PAS2035 compliance.
- ▸Installation oversight: site visits, quality inspections and sign-off against the design specification
- ▸Air-tightness testing pre- and post-installation to verify fabric performance improvements
- ▸Resident engagement throughout: pre-installation briefing, during-works support and post-occupancy guidance
- ▸Post-retrofit monitoring: energy consumption, internal temperature and humidity data collected for 12 months
- ▸Performance gap analysis: modelled vs actual energy performance with variance reporting
- ▸PAS2035 completion certificate and TrustMark lodgement on project sign-off
Home Energy Model — four headline metrics.
From 2027, HEM replaces EPC as the statutory energy assessment methodology. Every retrofit project is modelled against four headline metrics, each decomposed into contributing factors so you can see exactly which measures move the needle.
Fabric Performance
U-values and thermal bridging of the building envelope — walls, roof, floor, windows and doors.
Loft insulation, wall insulation, triple glazing, thermal bridging detailing, air-tightness
Heating System
Efficiency and carbon intensity of the primary heat source and its distribution.
Heat pump installation, emitter upgrades, smart controls, flow temperature optimisation
Smart Readiness
Capability to integrate smart energy management, monitoring and demand-side response.
Smart metering, zonal controls, battery storage, EV charge integration, demand-side response
Energy Cost
Modelled annual energy cost — the resident-facing affordability metric that drives fuel poverty outcomes.
All measures feed into this composite metric — fabric and heating improvements have the largest impact
Standards and references we work to.
Retrofit Standard
The British Standard for retrofitting dwellings for improved energy efficiency — the core framework we follow.
Installation Standard
Companion standard specifying requirements for retrofit installation quality and process management.
Quality Framework
Government-endorsed quality scheme under which all PAS2035 retrofit projects must be registered.
Energy Model
Home Energy Model — the four-metric replacement for EPC, assessing fabric, heating, smart readiness and cost.
Funding
Social Housing Decarbonisation Fund — the principal grant route for social housing retrofit.
Grant Schemes
Energy Company Obligation and Great British Insulation Scheme — supplementary funding pathways.
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